Implementation potential

The evaluated urine dehydration system is flexible enough to be added to existing sanitation systems. The installation is not complex and no extra piping is required, but a service provider would be needed to transport and process the end product. The system is flexible to expand as a city is growing – one does not need to wait for a sewage network to have a hygienic sanitation system, instead the treatment is on-site and supports the polluter pays principle.

Urine-diverting toilets collect urine separately from faeces during excretion and such toilets are already available on the market as urine-diverting flush toilets or urine-diverting dry toilets. In the present study’s system, either type of these user interfaces could be used. Following excretion, urine would be diverted into a containment unit located close to the seat, within the toilet room. The function of the containment unit is based on the principle of alkaline dehydration whereby the mass of urine is reduced by 95 % and nutrients are simultaneously retained in an alkaline dehydration medium.

Once the dehydrating medium is saturated (pH < 10.5), the unit (potentially a plastic bag or box) would be removed and enter the recycling system in a similar way that household organic waste is removed and entered into a recycling system. A service provider could operate this system in a similar way as for municipal solid waste collection. The quantity of the saturated medium per person per year would be approximately 40 kg, which is minimal compared to municipal solid waste production, 480 kg person^–1yr^–1, (Hoornweg and Bhada–Tata, 2012) and the same size as the food waste collected in Swedish municipalities with separate food waste collection (Avfall Sverige, 2017).

Faecal management was not the focus of this thesis, however its management should briefly be considered. In the urine-diverting flush toilet, faeces along with flush water could be transported by pipes for treatment at the wastewater treatment plant. The wiping material and/or anal cleansing water would be added to the faeces through the same user interface. In the urine-diverting dry toilet, faecal dehydration in another containment unit beside the urine dehydration unit could be used (Tilley et al., 2014). Diverting urine and faeces away from the wastewater treatment plant could greatly reduce the complexity and costs of operation since approximately 88 % N and 67 % P would already be removed (Jönsson et al., 2005). The saturated medium from the urine dehydration (and dried faeces) process would be taken to a treatment and conversion plant. The saturated medium (urine) would then be pelletised and packaged to be sold as a fertiliser that is easy to transport and store until use.

 The alkaline dehydration system enables the concentration of nutrients in urine, eliminates the need for liquid disposal and produces a safe dry fertiliser within four days of storage at 20 °C after the last use.

 For the variants of media and dehydration conditions investigated, 95 % of the liquid was reduced by volume. Including the ash, the volume reduction ranged from 75 to 90 % depending on the ratio of dehydration medium used.

 N retention ranged from 64 to 90 % and complete recovery of P and K were achieved.

 Increasing the urine dosing while decreasing the initial medium load yielded higher dehydration rates. The optimal conditions found for dehydration were an initial medium load of 100 g ash and loading urine of 80 ml (repeated after >90% would be evaporated) at 41 °C with an air flow rate of 0.5 L min^–1.

 Dehydrating urine at higher temperatures (35 to 65 °C) offers flexibility to either increase capacity or decrease the required surface area of ash.

 Sustaining alkalinity is necessary to ensure the effectiveness of the process, hence the pH of the dehydration medium is what determines the time range within which dehydration can be performed.

 For guideline standards on re-use, such as from WHO (2006) that specify a maximum acceptable concentration in the end-product, urine diversion is actually an effective treatment in its own right for helminths (Ascaris).

7 Conclusions

 Installing such a dehydration system in new urine-diverting and existing dry toilets could greatly simplify transportation and fertiliser application since there would be no liquid disposal.

 The urine can be collected, contained, treated and reduced within the same collection container to produce a dry end product with a high N-P-K concentration (up to 7.8–2.5–10.9 % of N, P and K), i.e. a fertiliser with similar concentration as existing mineral fertiliser and thus have a potential monetary value of US$ 115 tonne^{− 1}.

The performance of this system in terms of efficiency and hygiene safety should be assessed under the following conditions:

 cold (4 °C) and extremely cold (-20 °C) conditions

 on a larger scale

o households of four to six people o public toilets with high peak loads

 confirmation of urease inactivation with faecal contamination in the dehydration medium

 assessment of the inactivation of microorganisms in saturated ash in a more humid environment (> 70 %) and at lower temperatures (< 20 °C)

 assessment of the inactivation of other pathogens, such as Vibrio cholera and Cryptosporidium oocysts

Further research and development to be explored:

 how much could the urine:medium ratio increase if the dehydration is done with closed gas loop, where the gas is first dried and then heated by a dehumidifier

 whether ion exchange resin can be used effectively

 whether there are other resins or chemicals that can increase the pH of urine efficiently to enable pipe transport

 energy requirements compared to other technologies

 user acceptance of this kind of toilet technology

 how faeces could be managed alongside this urine treatment technology

 fertiliser potential and possible effects of the fertilisers salinity (should the salinity be decreased with ion-exchanger?)

 how the dry fertiliser product can be turned into strong granules similar to present commercial mineral fertilisers.